Hadith 3:190

Narrated 'Aisha:

Allah's Apostle used to fast till one would say that he would never
stop fasting, and he would abandon fasting till one would say that he
would never fast. I never saw Allah's Apostle fasting for a whole
month except the month of Ramadan, and did not see him fasting in any
month more than in the month of Sha'ban.